1. Field of the Invention
The present invention relates to a variable capacitor whose capacitance value is changed in accordance with a pressure or a displacement applied thereto from the outside, a position indicator using the variable capacitance, and an input device using the position indicator.
2. Description of the Related Art
In recent years, an input device has been used with a personal computer and the like. The input device is composed, for example, of a position indicator and a position detector. In this case, the position indicator is formed as of a pen type. The position detector has an input surface to which a pointing operation, characters, figures and the like are inputted by using the position indicator.
A variable capacitor as described in Japanese Patent Laid-Open No. Hei 4-96212 (hereinafter referred to as Patent Document 1) has been used in a pen-pressure detecting portion of the position indicator. The variable capacitor as described in Patent Document 1 includes a first electrode and a second electrode. The first electrode is mounted to one surface of a dielectric. The second electrode is disposed on the other surface of the dielectric and has flexibility. The variable capacitor includes a section for separating the second electrode and the other surface of the dielectric by a small space, except for a portion of the second electrode and the other surface of the dielectric, and a section for applying a relative pressure or displacement between the second electrode and the dielectric.
FIGS. 21A and 21B are schematic views each showing a concrete construction of a conventional variable capacitor. FIG. 21A is a schematic view showing an initial state in the conventional variable capacitor, and FIG. 21B is a schematic view showing a state in which a pressure is applied to the conventional variable capacitor.
The variable capacitor 200 includes a dielectric 201 having approximately a disc-like shape, a first electrode 202, and a second electrode 203. In this case, the first electrode 202 is mounted on one surface 201a of the dielectric 201. The second electrode 203 is disposed on the other surface 201b opposite to the one surface 201a of the dielectric 201. The second electrode 203 has flexibility and is disposed on the other surface 201b of the dielectric 201 through a ring-like spacer 204. In addition, a rod-like core body 210 is provided on a side of the second electrode 203 opposite to the dielectric 201 side through an elastic body 205.
A first terminal 206 is provided on the one surface side of the first electrode 202. The first terminal 206 is composed of a disc-like flange portion 206a, and a lead portion 206b extending approximately from a center of one surface of the disc-like flange portion 206a. When a pen pressure is applied to the variable capacitor 200, the flange portion 206a comes in contact with one surface of the first electrode 202 to be electrically connected to the first electrode 202.
A second terminal 207 is provided in an end (edge) portion of the second electrode 203. The second terminal 207 is composed of a disc-like flange portion 207a and a lead portion 207b extending approximately from a center of one surface of the flange portion 207a similarly to the case of the first terminal 206. When the pen pressure is applied to the variable capacitor 200, the flange portion 207a is in contact with an end portion of one surface of the second electrode 203 and is electrically connected to the first electrode 203.
In the variable capacitor 200, in a state in which neither the pressure nor the displacement is applied to the core body 210 (initial state), a small space is defined between the other surface 201b of the dielectric 201 and the second electrode 203 by a spacer 204. As shown in FIG. 21B, when the pressure is applied to the core body 210, both of the elastic body 205 and the second electrode 203 are depressed by the core body 210 to be elastically deformed. As a result, the second electrode 203 comes in contact with the other surface 201b of the dielectric 201. When a contact area between the second electrode 203 and the other surface 201b of the dielectric 201 is increased, a value of an electrical capacitance defined between the first and second terminals 206 and 207 is increased. As a result, a change in value of the electrical capacitance defined between the first and second terminals 206 and 207 is detected, thereby detecting the pressure (pen pressure) applied to the core body 210.